Conventional Darlington Amplifiers are often employed in a 3 terminal package (i.e., a SOT-89 package) or other 3 terminal style transistor package. Such amplifier packages are attractive because they typically have a small outline, a low cost, and a user-friendly implementation. Conventional 3-terminal transistor style packages typically allow one RF input, one RF output, and one reference ground terminal. The limited number of terminals imposes a constraint for bias solutions in Darlington amplifiers.
Referring to FIG. 1, a circuit 10 illustrating a conventional Darlington gain block biasing scheme is shown. In the circuit 10, an off-chip bias set Resistor RDC is used to set a nominal bias current Icc presented through a gain block 12. The gain block 12 presents a signal OUT in response to a signal IN and the current Icc. Typically, the more voltage drop across the resistor RDC, the better the stability of the bias current Icc over temperature and supply variations.
The DC bias set resistor RDC sets the total current bias of the circuit 10. However, the DC bias set resistor RDC also creates a voltage drop, reducing the output voltage and RF headroom provided to the Darlington output stage.
The external DC bias set resistor RDC has one or more of the following disadvantages (a) a limited output voltage swing and power output, (b) a limited DC bias ramp up with input power (inhibiting class B operation), and/or (c) the need for a user to provide the external DC bias set resistor to set the bias of the amplifier.
Referring to FIG. 2, a more detailed diagram of the circuit 10 is shown. The larger the voltage drop across the resistor RDC, the lower the voltage applied to the collectors of the transistors Q1 and Q2 and the lower the value of the signal OUT. The reduced voltage swing of the signal OUT affects saturated output power, 1 dB compression, and output IP3 (third order intercept point).
Moreover, for class AB, B, and C operation (e.g., where the current Icc increases with input power), the resistor RDC will drop even more voltage, reducing the maximum output power capability. Conventional bias schemes therefore need the user to optimize the value of the resistors RDC in order to meet specific design criteria. Conventional setting the bias involves a trade-off between DC stability and radio frequency (RF) performance.
Referring to FIG. 3, a diagram illustrating the simulated current-voltage characteristics of a conventional Darlington bias circuit 10 is shown. A bias variation of 20 mA at 5V over a −25° C. to 25° C. temperature change is shown.
Referring to FIG. 4, a diagram illustrating a measured current-voltage characteristics of conventional bias of the circuit of FIG. 2 is shown. A bias variation of ˜8–9 mA at 5V over a 0 to 85° C. temperature change is shown. The instantaneous slope of the I-V curve at 5V illustrates bias sensitivity to the supply voltage.
Referring to FIG. 5, a diagram illustrating measured S-parameters over temperature comparison of the convention approach is shown. The gain changes by less than 0.5 dB over an 85 C. temperature (i.e., acceptable temperature gain variation).
It would be desirable to have a fully self-biased amplifier that does not need an external bias set resistor RDC where the amplifier maintains or improves (i) output voltage headroom and RF swing, (ii) tolerance to supply and temperature variations, (iii) enables class AB, B, C dynamic bias operation, and/or (iv) may be implemented in a 3-terminal package (e.g., 5512) traditionally having one RF input, one RF output, and one ground.
It would also be desirable to implement a self-bias solution for a Darlington amplifier that may (i) be implemented in a 3-terminal transistor style package such as the SOT-89, (ii) eliminate the need for an external DC bias resistor, and/or (iii) allow high output voltage headroom and output power.
It would also be desirable to implement a self-bias solution that (i) has a lower sensitivity to temperature and supply voltage variation, and/or (ii) may be monolithically integrated with the Darlington amplifier to reduce size, cost, and/or complexity of integration.